Photographic reproduction employing both sharp and unsharp masking



June 17,1969

Filed Sept. 14, 1966 D. PHOTOGRAPHIC R-EPRO J. KYTE Sheet MAXIMUM SELECTOR 34 3 8 I 6 2 SUBTRAET 44 *5" 1.2

45 MAXIMUM I SELECTOR A ADD 5 M l NIMUM T SELECTOR 49 36 1,0

M SUBTRAU 5 41 43 M NIMUM i SELECTOR INVENTOR DEREK 0. KY TE June 17, 1969 J, KYTE 3,450,830 PHOTOGR IC REPROD TION EMPLOYING BOTH RP AND UN RF MASKING Sheet 2 of5 Filed Sept. 14, 1966 TEST TRANSPARENCY M WYWMW'CW M WBYBMIBCBQB FIGBB Y SIGNAL F lG. 3C M SIGNAL FIG. 30 c SIGNAL MAXIMUM 0F Y,M AND L SIGNALS rxvsuron DE EEK J. KYTE 5 wwhowvamev June 17, 1969 D. J. KYTE 3,450,830

PHOTOGRAPHIC REPRODUCTION EMPLOYING BOTH SH ARP AND UNSHARP MASKING Filed Sept. :14 1966 Sheet 3 of 5 .M wvwmwcwg w sjvsmece FIG. 3F w SIGNAL FIG. 36 MAXIMUM OF Y.M, C AND w SIGNALS OUTPUT OF SUBTRACTION CIRCUIT 42 zuvzmon 'DEPEK d. KYTE aawm'am g 3A.

' June '17, 1969 H 3,450,830

. D. J; KYTE PHOTOGRAPHIC REPRODUCTION EMPLO G BOT I SHARP- AND UNSHARP'MASKIN Filed Sept. 14, 1966 Sheet '4 015 .MINIMUM OF Y/MIANDC SIIGNALS MINIMUM 0% Y,M;c, AND w SIGNALS OUTPUT OFS UBTRACTION CIRCUIT 43 FIG. 3N

INYENTOR DEREK d. KYTE June 17, 1969 ,'D. J. KYTE 3,

PHOTOGRAPHIC REPRODUCTION, EMPLOYING BOTH SHARP AND UNSHARP MASKING Filed Sept.. 14,1966

Sheet 5 015 ruvzuron DEREK d. KYTE 3,450,830 PHOTOGRAPHIC REPRODUCTION EMPLOYING BOTH SHARP AND UNSHARP MASKING Derek J. Kyte, 117 Valley Road, Chorleywood, England Filed Sept. 14, 1966, Ser. No. 579,298 Claims priority, application Great Britain, Sept. 20, 1965, 40,081/ 65 Int. Cl. H04n 5/38, 5/44 U.S. Cl. 178-5.2 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to apparatus for the photo-electric scanning of coloured originals and the simultaneous reproduction of colour separation images on film or other recording media. The invention is particularly concerned with the improvement in the apparent sharpness of such images by a combination of optical and electronic means.

In photographic processes, particularly in those involving colour masking, it has been known as unsharp masking. This technique has been fully described by Yule in US. Patent 2,455,849. The basis of the technique is the combination of two photographic records of an original, one of which is sharp and the other of which is less sharp. The two records must be of opposite senses, i.e. one must be a negative and the other a positive. If two such records are superimposed, the contrast or density range of the combination will be reduced more in broad areas of tone than in regions where sharp tonal changes occur. To the human eye, such a combination will appear to be sharper, and small details will appear more easily discernible than in a sharp record of equivalent over-all contrast.

In the well known methods of photographic colour correction known as colour masking, where for example negative records of a coloured original made through certain colour :filters are masked by positive records made through filters of other colours, it is quite usual for the mask to be made slightly unsharp. In this way, it is possible to achieve both colour correction and unsharp masking in one operation. The direct application of this method to electro-optical scanning and reproduction apparatus has been described by Yule in British Patent 712,499. In this patent was disclosed a method of optically scanning a coloured original through a filter of one colour to produce an electrical signal representative of finely detailed information on the original, simultaneously scanning the original through another filter to produce a signal representation of the coarser detail on the original, and the electrical combination of these signals to produce a further signal representing both a colour corrected and an unsharp masked record of the original.

This method has the major disadvantage that the degree of improvement in detail is so closely linked with the degree of colour masking that independent control of the two cannot be achieved. An alternative system was disclosed in British Patent 891,978. Here the electrical signal responsible for unsharp masking was derived independently of any colour correction process and it could States Patent 0 be applied in a desired degree to any or all of the colour separations without its action being affected by the degree of colour correction applied. A particular feature of this system was that the sharp and unsharp signals were both representative of the luminance of the original, i.e. to the approximate brightness of the original as seen by the human eye. Thus the unsharp masking would be most effective at regions where colours which appeartdark to the eye (e.g. reds, warm blues, blacks, etc.) were bounded by colours which appear light (e.g. yellows, light greens, etc.). However, when for example, originals were being reproduced which comprised regions of warm colours (e.g. reds, oranges) which appear dark to the human eye bounding, or bounded by, light greys or colours which appear light to the eye, intermediate haloes occurred. A typical example might be a picture of a basket of red cherries, many of which catch the reflection from a nearby source of light and, as a consequence, have almost white highlights on their surfaces. Because the unsharp masking signal is derived from luminance signals, it will be particularly effective at the boundaries between the red cherries and the highlights on their surfaces. 'Now if unsharp masking is to be effective, it is most important that it is applied to the cyan and black separations, since these colours are mainly responsible for making the final reproduction appear sharp and contrasty to the human eye. However, in both these separations the differences in tonal value between the red cherries and the highlights will be small (since red contains zero or very little cyan and black) and thus at the tonal junctions there will be only a small change in density.

The effect of luminance unsharp masking will be to exaggerate this change, so that a dark halo appears on the red side of the junction and a light halo on the white side. In the final printed reproduction, this will result in a dark ring appearing around the highlights on the cherries. Furthermore, when reproducing originals in which the flesh tones are rather red, a common fault with duplicated transparencies, the effect of luminance unsharp masking on such originals can be to exaggerate the blemishes on flesh and given an unpleasant sore appearance to the printed reproduction.

A practical difiiculty encountered with certain of such systems is that it is necessary for the photo-cells (or photo-multipliers) and the associated electrical circuits to be very exactly balanced with respect to colour. If this is not the case, then the unsharp masking signal may be other than zero when scanning areas of the original of uniform colour. If such is the case, then the unsharp masking signal will alter the colour balance of the reproduction.

The unsharp masking signal may also contain noise which will be added to the main separation signals in the computer and will increase the signal to noise ratio.

It is common knowledge that the sharpness and detail of a coloured reproduction depend mainly on the neutral content of the picture. Use is made of this fact in colour television where the colour signals are transmitted over narrow bandwidth channels whilst the luminance or neutral content signal is transmitted over a wide bandwidth channel.

In the application of unsharp masking to colour separations, therefore, it is sufiicient if the effect is predominantly confined to improving neutral contrast as opposed to contrast between colour hues. This is desirable also from other points of view, in particular to avoid the problem with warm reds, etc. as discussed above, and to compensate for the reduction of neutral detail contrast brought about by the process of undercolour removal.

It can be seen, therefore, that an ideal method of producing an unsharp masking signal would be to combine a sharp black separation with an inverted unsharp black separation. The resultant signal would be significant only in neutral details and could be used to increase the contrast of such details on any or all of the colour separations.

The basic method of producing a black separation electronically consists of selecting at every instant of time that one of the three colour separation signals which represents the least amount of ink. This principle is well known in the art and needs no further discussion here. Suffice to say that the three colour separation signals are derived primarily from three photo-cells or photo-multipliers, each of which has a different colour filter in front of it, and each of which receives part of the light resulting from the scanning of a small element of a coloured transparency or reflection original.

To produce an unsharp black separation in an identical fashion, it would be necessary to have three further photo-multipliers and colour filters, each of which receives part of the light derived by scanning a larger area of the original picture. In addition, further electronic circuits would be needed to amplify the photo-electric signals and to derive from them that corresponding to the least ink.

Apart from the added complexity and expense of such an arrangement, there would be considerable difiiculty in matching the three unsharp photo-cells, filters and associated circuits so that their response is identical with that of the three sharp systems.

It is an object of the present invention to provide means for improving the contrast of substantially only the neutral details on electro-optically produced colour separations, such means being independent of any colour correction circuits. It is a further object of the invention that the extra circuitry and optics required should be minimal, that the exact colour sensitivity of any additional photo-electric device should not be critical, and that the application of the resultant signal to improve detail contrast should not substantially affect the signal/ noise ratio in the main colour separation channels. The invention will now be described with reference to the accompanying drawings, of which:

FIG. 1 represents diagrammatically one form of electro-optical colour separation device incorporating the 1nvention.

FIG. 2 shows in block diagram form the electronic circuitry associated with the application of the invention.

FIGS. 3A-3P show various signal wave-forms within the electronic circuits.

FIG. 4 shows one form of actual circuit performing the functions shown diagrammatically in FIG. 2.

Referring to FIG. 1, a transparent scanning cylinder 1 and an exposing cylinder 2 rotate in synchronism and are driven by a constant speed motor (not shown). A colour transparency is held on the outer surface of cylinder 1 and a beam of light from lamp 3 and lens 4 is projected through the drum and the transparency mounted thereon. The emergent light is collected by lens 5, which is disposed so as to focus an image of the illuminated point of the transparency onto the plane of an aperture plate 6. This plate contains a small hole which will normally be approximately in the centre of the area of plate 6 illuminated by the image. The light emerging from the hole is collimated by lens 7 and split into three beams by the partially reflecting surfaces 8, 9 and the fully reflecting surface 10. After passing through colour filters 11, 12 and 13 (normally coloured blue, green and red), the three beams fall on photocells 14, 15, 16. The electrical signals from these photo-cells are fed via suitable amplifiers 17, 18 and 19 to colour correction and black printer circuits 20. Many such circuits are known in the art and their exact nature forms no part of this invention. The outputs of the circuits 20 will normally be four in number and will be approximately representative respectively of the amount of yellow, magenta, cyan and black inks required to reproduce the colour of the scanned element of the transparency. In FIG. 1, these four outputs are designated by the letters Y, M, C and B respectively.

One of the signals, for example the magenta signal, is shown connected to the circuit 21, whose function is explained below. The output of circuit 21 is fed to the tonal and contrast adjustment circuits 22, the output signal from which controls the brightness of a glow modulator lamp 23. The light from this lamp is focussed by lens 24 to form a small spot on a sheet of light sensitive film held on the cylinder 2.

Whilst the cylinders revolve, they (or the scanning and exposing optics) are made to move transversely by a feed mechanism (not shown) so that the whole transparency is scanned in a spiral fashion and the film cylinder 2 is virtually simultaneously exposed point by point and line by line. After suitable processing, the image on the film will be representative of the amount of magenta ink, for example, which in conjunction with other inks is required to reproduce the colours of the original transparency. A printing plate may be made from this film by any of the usual methods.

By repeating the scanning process with circuit 21 connected in turn to the yellow, cyan and black outputs of circuit 20, successive films may be produced which represent the amounts of yellow, cyan and black inks required to reproduce the original. Alternatively, four groups such as circuits 21, 22, glow lamp 23 and lens 24 may be provided, together with four exposing cylinders, so that all four colour separations may be produced simultaneously.

The additional components for adding contrast mainly to neutral details, consist of partially reflecting surface 25, filter 26, photo-cell 27 and the circuits 28, 29 and 21. It is a necessary condition for these circuits to be effective that the photo-cell 27 receives light from a larger element of the transparency than do the photo-cells 14, 15 and 16. This condition will be met if the image of the illuminated element of the transparency on aperture plate 6 is larger than the hole in this plate. Suitably, the image would be two or three times the diameter of the hole.

The photo-cell 27 must have a sensitivity over virtually the whole visible range of wavelengths and this sensitivity should be substantially equal in the three regions of the visible spectrum to that defined by the blue, green, and red, colour filters in combination with the photocathodes of photo-cells 14, 15 and 16. This is easy to achieve with photo-cells having S11 photo-cathodes by placing a light orange filter in front. The object of filter 26, therefore, is to achieve this condition. Its exact colour will, however, depend on the type of photo-cathode possessed by photocell 27. Thus photo-cell 27 is making a white light scan, in contra-distinction to photo-cells 14, 15 and 16 which make blue, green and red light scans respectively.

The electrical signal from photo-cell 27 is amplified in circuit 28 which is preferably of exactly the same type as circuits 17, 18 and 19. The output from circuit 28 designated by W, and the Y, M and C outputs from circuit 20 are all fed into the circuit 29, in which a special signal is derived for the purposes of improving the contrast of small details. The method of operation of this circuit will be described with reference to FIG. 2, which shows the main parts of circuit 29 in block diagram form.

The input signals on leads Y, M, C and W derived from the blue, green, red and white light photo-cells respectively are fed into circuit 34, which is a maximum signal selector. Thus the output of circuit 34 on conductor 38 is equal at every instant to whichever one of the four inputs is the greatest in amplitude. It is assumed that the signal amplitude in all cases increases with increasing transmittance of the scanned transparency (i.e. light areas give high signals, dense areas give low signals).

The signals Y, M and C are also fed into maximum signal amplitude selector 35. The greatest of these therefore appears at the output of this circuit on conductor 39. It will be apparent to those skilled in the art that the signal on conductor 39 is representative of the neutral content of that part of the transparency being scanned. In other words, the signal on conductor 39 is a black separation. Conductors 38, 39 are connected to substraction circuit 42, which is arranged to subtract the signal on conductor 39 from that on conductor 38.

If any of the signals Y, M or C are greater than W, then the output of circuit 34 on conductor 38 will be identical with the output of circuit 35. In this case, the output of the subtraction circuit 42 will be zero.

If the signal W is greater than Y, M and C, then the output of circuit 34 will be W. In this case, therefore, the output of circuit 42 will be the result of subtracting the largest of the Y, M and C signals from the W signal.

Similarly, the signals Y, M, C and W are fed to a minimum selector circuit 36. The signal at the output of this circuit on conductor 40 will be equal to the least of the the four input signals. The signals Y, M and C are also fed to a minimum selector circuit 37, the output of which on conductor 41 represents the least of the three inputs. Conductors 40, 41 are connected to subtraction circuit 43 which is arranged to subtract the signal on 41 from the signal on 40.

If any of the signals Y, M or C are less than W, then the outputs of both circuits 36 and 37 will both be equal to the least of the three signals Y, M, and C and thus the output of circuit 43 will be zero. If the signal W is less than Y, M and C, then the output of circuit 36 will be W and the output of the subtraction circuit 43 will be the result of subtracting the least of Y, M or C from W.

The outputs 44, 46 of the two subtraction circuits 42 and 43 are fed via separate adjustable potentiometers 45 and 47 to the addition circuit 50, the output of which appears on conductor 51.

Reverting to FIG. 1, the output signal on conductor 51 is added to the main separation signals, either One at a time in turn in a single circuit 21, or simultaneously in respective circuits 21.

In order to understand how the above circuits afiect the reproduction of neutral details, the time response of the various circuits will be explained when scanning a particular sequence of both coloured and neutral details. It will be imagined that a transparency is being scanned which contains a number of parallel strips of pure colours and neutrals as shown in FIG. 3A. It 'will be supposed that the width of the strips is of the same order as the effective diameter of the scanned element seen by photocells 14, 15 and 16 and that the diameter of the element seen by photo-cell 27 is about 2 times as large.

It will also be supposed that the direction of scan is at right angles to the aforesaid strips and that the colour correction circuits 20 have been adjusted to give a virtually perfect colour response defined as follows:

When scanning white, the Y, M and C signals are equal and high (the so-called white level).

When scanning black, the Y, M and C signals are equal and low (the so-called black level).

When scanning pure yellow, the Y signal is equal to the black level and the M and C signals are equal to the white level.

When scanning pure magenta, the M signal is equal to the black level and the Y and C signals are equal to the white level.

When scanning pure cyan, the C signal is equal to the black level and the M and Y signals are equal to the white level.

These requirements are not essential to the operation of the system, but make the description simpler. The waveforms shown in FIG. 3 are constructed by joining points by straight lines and are only for illustrative purposes. The actual waveforms obtained in practice will be composed mainly of curves, the shape of such curves depending on the shape of the scanned picture elements and the amplitude response curves of the electronic circuits.

FIG. 3A shows a test transparency consisting of narrow yellow, magenta cyan and middle grey strips on a white background followed by similar strips on a black background. FIGS. 3B, 3C and 3D show the amplitude of the Y, M and C signals respectively when such a transparency is scanned from left to right. In these three figures and all subsequent figures to FIG. 3P, the horizontal axis represents time and the vertical axis represents amplitude in arbitrary units.

The instantaneous maximum of the Y, M and C signals is shown in FIG. 3E, and this will be the output signal from the circuit 35 in FIG. 2. It will be seen that this signal is at the white level, or attains the white level, for all coloured regions, whereas in neutral areas its amplitude corresponds to the strength of grey present in the original.

FIG. 3F shows the W signal. Because this signal is derived from the scanning of a larger element of the original, the responses to scanned details stretch over a longer period of time than those illustrated in FIGS. 3B, C and D. Moreover, in the case of small netural details which are smaller than the scanned element, the amplitude change when traversing such details is less than for a scanning element of smaller size.

In FIG. 36 is shown the instantaneous maximum of the Y, M, C and W signals. It will be seen that the peak response to coloured areas is identical with that of the signal shown in FIG. 3B. Thus, in general, the output of the subtraction circuit 42, which subtracts the signal of FIG. 3G from that of FIG. 3B, will be zero in pure colours. This output is shown in FIG. 3H. Assuming for the moment that the output of the subtraction circuit 42 is zero, the output of the adding circuit 50 on conductor 51 will be similar to the output of subtraction circuit 42 except that the amplitude may be reduced by the setting of potentiometer 45. This signal is added to the main separation signal in the addition circuit 21. If potentiometer 45 is set so that its slider is at earth potential, there is no effect on the separation signal and it will have one of the forms shown in FIGS. 3B, C, D or E. In particular, the black separation signal will be similar to that shown in FIG. 3B. If this black separation signal is being used for exposing, i.e. it is connected to the input of circuit 21, and if now the potentiometer 45 is rotated so that its slider is no longer at earth potential, then the output of circuit 21 will be a modified black signal as illustrated in FIG. 3].

By comparing this figure with FIG. 3E, it can be seen that:

(a) when a grey detail stands against a white background, the grey detail is made darker;

(b) where a grey detail stands against a White background, the background adjacent to the detail is made darker;

(c) where colours stand against a white background, there is no effect;

(d) where colours stand against a dark background, the

background adjacent to the detail is made darker.

Thus the contrast of neutral details, and the contrast between neutral areas and coloured areas, has been increased by darkening of neutral details or neutral boundaries. In no case has the peak response to any colour been altered, even though the coloured details are smaller than the larger scanned element viewed by the white light photo-cell 27.

The remainder of the circuits shown in FIG. 2, i.e. circuit 36, 37 and 43 operate in an analoguous manner to those discussed above.

The output of circuit 37 represents the instantaneous minimum of the signals Y, M and C and is illustrated in FIG. 3K. The output of circuit 36 represents the instantaneous minimum of the Y, M, C and W signals and this is illustrated in FIG. 3L. The subtraction circuit 43 subtracts the output from circuit 36 from that of circuit 37 and its output is shown in FIG. 3M.

Assuming that the output from subtraction circuit 42 is zero, the effect of the output from subtraction circuit 43 on a black separation signal is shown in FIG. 3N. By comparing this figure with that illustrating an unmodified black separation (FIG. 3E), it can be seen that:

(a) the grey detail on a dark background is made lighter;

(b) where a grey detail is against a light background, the background adjacent to the detail is made lighter;

(c) where coloured details stand against a dark background, there is no effect;

((1) where coloured details stand against a light background, the background adjacent to the details is made lighter.

Thus, in this case, the contrast of neutral details and the contrast between coloured details and neutral areas has been increased by lightening neutral details or neutral boundaries. Again, the peak response to coloured details has not been altered.

When the outputs from both subtraction circuits 42 and 43 are being utilised, these outputs are added in circuit 50 to produce a composite signal on conductor 51. The effect of such a composite signal on a black separation signal is illustrated in FIG. 3P. By comparing this signal with the unmodified black signal shown in FIG. 3B, it can be seen that:

(a) where a grey detail stands against a light neutral background, the grey detail is made darker whilst the background adjacent to the detail is made lighter;

(b) Where a grey detail stands against a dark neutral background, the grey detail is made lighter whilst the background adjacent to the detail is made darker;

(c) where coloured details stand against a lighter background, the background adjacent to the details is made lighter;

(d) where coloured detail stand against a dark background, the background adjacent to the details is made darker.

The overall effect of the circuits described, therefore, is to improve the contrast of neutral details and of boundaries between neutral and coloured areas Without substantially affecting the coloured details themselves.

In the above description, only pure neutral areas and pure colours have been considered. When areas of dirty colours are considered (i.e. colours which contain a proportion of grey), then the effect of the detail contrast circuit is found to be proportional to the grey content of such colours.

In the above description, the illustration of the effect of the detail contrast circuits was confined to the black separation. It is on this assumption that the visual effect on a printed result will be most noticeable, since black ink is darker to the human eye than the coloured inks. However, it is often advantageous to utilise the detail contrast signal also when exposing the yellow, magenta and cyan separations. It is preferable that the amplitude of this signal-as determined primarily by the setting of potentiometers 45 and 47should be roughly the same on all three separations to avoid the false colouration of small details.

From the description above, it will be seen that this system of improving detail contrast does not rely on the exact balancing of the colour sensitivity of one photocell with that of another. Providing that the white photo-cell 27 has substantially equal sensitivity over the three bands of wavelengths defined by photo-cells 14, 15, .16 in conjunction with filters 11, 12 and 13 respectively, then the signal from this photo-cell will be in effect the average of the three signals from photo-cells 14, 15 and 16, when areas of wave-form colour larger than the large scanned element are scanned. Thus for any colour, the response of the white photo-cell 27 will be less with respect to the white level than the greatest of the responses from the blue Y, green M or red C photo-cells and will be greater with respect to the white level than the least of the responses from the blue, green or red photo-cells.

Should the sensitivity of photo-cell 27 depart slightly from the ideal defined above, then the effect will be that for certain colours very near neutral, the response of photo-cell 27 may not be less than the greatest or more than the least of the three responses from the blue, green and red cells. Since the colour concerned is near neutral, however, the response of the white photo-cell and both the maximum and minimum responses of the blue, gr en and red photo-cells will be so close together that the output of the subtraction circuits 42 and 43 will be near zero anyhow. Thus the effect on the main separation signal will be insignificant. Such effects are in any event minimized by the noise suppression circuits described below.

Referring again to FIG. 2, it has already been explained that when scanning a broad area of colour, one or other of the Y, M and C signals will be greater than the W signal and one or other will be less than the W signal. The outputs of circuits 34 and 35, therefore, will be equal and since they are derived from the same input signal (whichever of the Y, M and C signals is the greater), any noise on these outputs will be correlated. This noise will, therefore, cancel in the subtraction circuit 42. The same agrument can be applied to the output signals from circuits 36 and 37. Thus, when scanning coloured areas larger than the large scanning element, the detail contrast system will not add appreciable noise to the main separation signal.

When scanning large (in the above sense) neutral areas, the Y, M, C and W signals will all be equal. The output of circuit 35 will thus contain the instantaneous maximum of the noise signals on the Y, M and C inputs. The output of circuit 34, however, will contain the instantaneous maximum of the Y, M, C and W noise signals. These two outputs will not have fully correlated noise signals and thus there will in this case be some noise present at the output of subtraction circuit 42. Similarly, noise will be present at the output of circuit 43.

These noise signals can be eliminated by introducing threshold values in the circuits 34 and 36 so that in the former case, the W signal is only selected if it is greater than either the Y, M or C signals by a certain value, and in the latter case the W signal is only selected it it is less than the Y, M, or C signals by a certain value. The value of the threshold should be chosen to be greater than the peak noise voltage expected on the W signal plus the peak noise voltage expected on the Y, M or C signals.

When large neutral areas are scanned, and the Y, M, C and W signals are of equal mean amplitude, the threshold value will mean that output from the circuits 34 and 36 will be derived only from Y, M and C signals. Thus the noise signals from circuits 34 and 35, and from 36 and 37 will be correlated and the outputs of subtraction circuits 42 and 53 will be substantially noise free.

As far as neutral and coloured areas larger than the larger scanned element are concerned, therefore, the detail contrast circuits will not appreciably affect the signal/ noise ratio of the main separation signals. When scanning details, of course, extra noise will appear but this will be confined to regions of the exposed separations which are very small and will not be visible to the eye, nor will they affect the printed result.

The introduction of threshold values as explained above will also prevent small differences in the colour sensitivity of photo-cell 27 from the ideal from affecting the separations. Moreover, the existence of thresholds will prevent the detail contrast system from Working when very small or very low contrast details are being scanned. This can be advantageous in preventing grain and dust spots on the original transparency from being exaggerated on the exposed separations.

Referring again to FIG. 1, it will be seen that here the Y, M and C signals fed to the detail contrast circuit 29 are taken from the outputs of the colour correction circuits 20. It is not essential that this is done. The system will operate satisfactorily if the Y, M and C signals from the outputs of the amplifiers 17, 18 and 19 are used instead. In this case, however, the increase in detail contrast may have a slight eifect on certain pure colour detailsparticula-rly the green, blue and violet details. The reason for this is that when scanning such colours, the basic photo-cell signals Will indicate that they contain an appreciable amount of grey. The effect is not great, and may be advantageous in some cases.

In FIG. 4, a circuit is shown which performs the functions shown in block form in FIG. 2. This particular circuit is designed for DC. operation and it is assumed in this case that the Y, M and C signals are available as DC. signals of both polarities. Thus the signals Y, M and C are negative going Whilst the signals Y, M and C are positive going. The signal W is only required as a negative going signal.

Diodes D1, D2 and D3 form a maximum signal selector so that the signal on conductor 1 is positive and equal to the maximum of the Y, M and C signals, less the voltage drop across a diode. These three diodes form the equivalent of circuit 35 in FIG. 2.

Diodes D4, D5, D6, D7 and D8 for-m another maximum signal selector. The signal conductor 3 is negative and equal to the maximum of the Y, M and C signals and the signal at the junction of D7 and D8, less the volts drop across one diode. The signal at the junction of D7 and D8 is equal to the W signal less the volts drop across D8. Thus D8 gives the threshold value referred to above for the suppression of noise in neutral areas.

The signals on conductors 1 and 3 are added by the network of resistors R1, R2, potentiometer P1, DC amplifier 50 and the feedback resistor R7. This is standard circuitry and needs no further explanation here.

Since the signals on conductors 1 and 3 are of opposite polarity, the process of addition corresponds to the subtraction circuit 42 in FIG. 2 where the inputs were assumed to be of the same polarity.

Diodes D9, D10 and D11, together with diode 17 and resistor R connected to a positive bias potential form a minimum selector circuit equivalent to circuit 37 in FIG. 2. The potential at the junction of resistors R3 and R5 is positive and equal to the least of the Y, M and C signals less twice the volts drop across a diode. The purpose of diode D17 is to make this circuit symmetrical, as far as the Y, M and C signals are concerned, with the second minimum selector described below.

The diodes D12, D13, D14, D15, D16 and resistor R6 connected to a n egative bias voltage form a second minimum selector. The signal at the junction of R4 and R6 is negative and equal to the least of (Y2V), (M2V), (C2V) and (W- V) signals, where V is the volts drop across a diode. The threshold value in this case is obtained by inserting a diode D16 in series with that part of the circuit concerned with obtaining the minimum of the Y, M and C signals.

The signals on conductors 4 and 6 are of opposite polarity and are added by the network R3, R4, potentiometer P2 and the DC. amplifier 50 and feedback resis- 10 tor R7. This addition process corresponds to the subtraction circuit 43 of the FIG. 2.

The output of the whole circuit is on conducor 51 and this may be added to the main separation signal as described earlier to improve the contrast of small neutral details. The amount of the effect may be controlled by potentiometers P1, which affects primarily dark details on a light background and P2, which affects light details on a dark background. Similar circuits may be devised for achieving the same functions, either with DC. or AC. signals. No mention has been made of the nature of the amplitude response of the circuitry to the original photocell signals. They may bear a linear relationship or a logarithmic relationship or any combination of these. With linear characteristics, the effect of the detail contrast circuits will be reduced where dark details against dark backgrounds are concerned relative to their effect on light details on light backgrounds With logarithmic relationships, the effect of the detail contrast circuits may be made to extend to dark details against dark backgrounds.

What I claim is:

1. Apparatus for the production of colour separation images from coloured originals comprising photo-electric sharp scanning equipment and associated colour separation equipment for producing electrical colour separation signals; photoelectric unsharp scanning equipment for producing electrical white signals; means for determining a first difference (if any) between the maximum individual signal amplitude occurring among the colour separation signals plus the White signal, and the maximum individual signal amplitude occurring among the colour separation signals alone; means for determining a second difference (if any) between the minimum individual signal amplitude occurring among the colour separation signals plus the white signal and the minimum individual signal amplitude occurring among the colour separation signals alone; means for determining the sum of said first and second differences; and means for adding said sum to each of the colour separation signals to form the signals to be used in the production of the colour separation images.

2. Apparatus as claimed in claim 1 and comprising means for ensuring that the White signal is selected as the signal of maximum, or minimum, amplitude, only if the white signal is greater, or less, than each of the colour separation signals by a predetermined amount.

3. Apparatus as claimed in claim 1 and comprising means for independently and variably adjusting the values of said differences before adding them together.

References Cited UNITED STATES PATENTS 2,981,792 4/1961 Farber 1785.2 3,100,815 8/1963 Drake l785.2 3,110,761 11/1963 Allen 1785.2

RICHARD MURRAY, Primary Examiner. J. MARTIN, Assistant Examiner. 

